If banks adopted ripple they would probably start their own private ledger and run it themselves. It's unlikely they would use the same ripple network everyone else does. In that case they wouldn't care about transaction fees.

Ripple transaction fees are negligible. If banks decided to use the current ripple network to process thousands of transactions the cost in fees would be insignificant anyway. They wouldn't need huge quantities of ripple for fees, or care what they cost..